1. Field of the Invention
The present invention relates to a backlight device being suitable for backlight for a liquid crystal display device and a liquid crystal display device having the same.
2. Description of the Related Art
FIG. 24 is a schematic cross sectional view showing an example of a conventional passive-type (a simple matrix type) liquid crystal display device. The liquid crystal display device 300 according to this example comprises a transmissive or transflective liquid crystal display unit 320 and a backlight device 310 disposed on the back surface of the liquid crystal display unit 320 (refer to Patent Documents 1 and 2).
The backlight device 310 is configured so that the light from an elongate light source 313 such as a Cold Cathode Florescent Lamp (CCFL) is introduced to an incident surface 312a (side surface) of the flat light guide plate 312 and is emitted from an emitting surface 312b (upper surface) of the light guide plate 312 facing the liquid crystal display unit 320.
The surface (lower surface) 312c opposite to the emitting surface 312b of the light guide plate 312 has a light reflectivity by forming a reflecting member 317 having a dot-shaped flat pattern or a number of white or reflective protrusions thereon.
A diffusion plate 314 is provided on the emitting surface 312b of the light guide plate 312, and two prism sheets 315, 316 are provided on the diffusion plate 314 in this order, thereby applying the light diffusivity, the uniformity of brightness distribution, and the light directivity. As shown in FIG. 25, each prism sheet is formed by forming a series of protrusions 318 having triangular sections and a series of grooves 319 having wedge-shaped, sections on a layer formed on a substrate. The two prism sheets 315, 316 are positioned such that the extended direction of the ridgelines of the protrusions 318 in one prism sheet is perpendicular to those of the protrusions 318 in the other prism sheet (the prisms are perpendicular to each other). Light component in a certain direction among light components emitted from the emitting surface 312b of the light guide plate 312 is transmitted through the prism sheet 315, is focused at a viewing angle having a certain angle range (for example, up to 70°); and is emitted as an emitted light component L21. Light components, in different directions are transmitted through the prism sheet 316, is focused at a viewing angle having a certain angle range (for example, up to 70°), and emitted as an emitted light component L22.
Recently, it has been suggested that a white LED (Light Emitting Diode) which is a point light source be employed as a light source of the backlight device.
[Patent Document 1]
Japanese Unexamined Patent Application Publication No. 11-500071 (corresponding U.S. Pat. No. 5,626,800)
[Patent Document 2]
Japanese Unexamined Patent Application Publication No. 2000-11728
However, the conventional backlight device 310 comprises the diffusion plate 314 and the two prism sheets 315, 316 which are provided separately from the light guide plate on the light guide plate 312. Therefore, there are problems in that the number of components increase, the structure thereof becomes complicated, and the thickness and cost thereof increases.
In addition, in the prior art, a cheap thin backlight device which can uniformly and brightly illuminate the display area of the liquid crystal display unit 320 cannot be realize.
Further, since the conventional liquid crystal display device 300 comprises the above-mentioned backlight device 310, the thickness of the entire device increases and the cost thereof increase.